1,720,957 research outputs found
A quantum Monte Carlo study of high pressure solid and liquid hydrogen
Get PDFHydrogen is the first element of the periodic table. As such, it is often regarded as the simplest one:
the non-relativistic hydrogen atom is a problem exactly solved in many textbooks; the hydrogen
molecular ion H 2 + and the diatomic molecule H 2 are, correspondingly, the first systems to be
considered when more than one nucleus is involved. As a thermodynamic system, its phase diagram
at low pressures is quite standard: at room temperature and ambient pressure, hydrogen is a
molecular fluid; upon cooling, it becomes a molecular solid; its critical point is T=33 K and P=1.3
Pa.
Nevertheless, even such a simple system becomes really interesting when pressure is increased by
several orders of magnitude. Speculations about the existence of a metallic solid state at 25 GPa
and 0 K temperature started with Wigner and Huntington; later calculations suggested that
this state could become a high-temperature superconductor . When experiments achieved the
predicted transition temperature, they did not find a metallic state; on the other hand, they found
a rich phase diagram, where several different solid phases exist. Nowadays, the quest for solid
metallic hydrogen at low temperature is still an on-going activity.
As temperature is increased above ≈ 1000 K, the system enters the liquid phase: it is important
to obtain an accurate equation of state at high temperature and high pressure, in order to model
the properties of gas giants, such as Jupiter and Saturn, which are mostly made of hydrogen and
helium. Metallic hydrogen, which is yet to be seen in the solid state, was experimentally measured
in the liquid phase.
Performing experiments at such high pressures is complicated; the information obtained is partial.
At low temperatures, the boundaries among the different solid phases can be drawn, but most
of their structural properties are still an open problem; at high temperatures, characterizing the
insulator-metal transition is hard because of large uncertainties and conflicting results.
Ab Initio simulations can be a valuable tool to complement and interpret experimental data; they
can also guide experiments with their predictive power. For condensed matter, Density Functional
Theory (DFT) is the method of choice to perform Ab Initio simulations at reasonable computational
cost. However, their predictive power for high pressure hydrogen is questioned due to several levels
of approximation which will be discussed in our work: in particular, the fact that DFT is plagued
by an uncontrolled approximation (the exchange-correlation functional approximation) will be
elaborated.
iiiIn this thesis, we will employ a different method to run Ab Initio simulations of high pressure
hydrogen at finite temperature: the Coupled Electron Ion Monte Carlo (CEIMC). We will discuss
how CEIMC, combining the Path Integral formalism to treat the nuclear degrees of freedom
and the Variational Monte Carlo (VMC) method to accurately compute electronic energies in a
Born-Oppenheimer framework, can perform finite temperature simulations without suffering from
the same kind of uncontrolled approximation which plagues DFT. We will then apply the method
to the low temperature, solid phase and to the high-temperature, liquid one. In the first case,
finite temperature simulations of different candidate structures for the various solid phases will
be performed, comparing CEIMC results with DFT ones. In the second case, the liquid-liquid
phase transition will be investigated, drawing attention to the relationship between molecular
dissociation and metallization; to do so, the system will be characterized across the transition with
the computation of relevant optical properties
Liquid–liquid phase transition in hydrogen by coupled electron–ion Monte Carlo simulations
Get PDFThe phase diagram of high-pressure hydrogen is of great interest for fundamental research, planetary physics, and energy applications. A first-order phase transition in the fluid phase between a molecular insulating fluid and a monoatomic metallic fluid has been predicted. The existence and precise location of the transition line is relevant for planetary models. Recent experiments reported contrasting results about the location of the transition. Theoretical results based on density functional theory are also very scattered. We report highly accurate coupled electron–ion Monte Carlo calculations of this transition, finding results that lie between the two experimental predictions, close to that measured in diamond anvil cell experiments but at 25–30 GPa higher pressure. The transition along an isotherm is signaled by a discontinuity in the specific volume, a sudden dissociation of the molecules, a jump in electrical conductivity, and loss of electron localization
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Get PDFWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
koamabayili/VECTRON-author-checklist: VECTRON author checklist
No full textWe have done our best to complete the author checklist relating to the use of animals in the hut study. Note that the objective for the hut study was to evaluate the IRS treatment applications for residual efficacy against Anopheles mosquitoes, including the local An. coluzzii mosquito population. Cows were only used to attract mosquitoes into the huts and no tests were carried out directly on the cows. The author checklist is intended for use with studies where experiments are carried out on animals, which is why we have had such difficulty in completing this for the hut study, as many of the questions do not relate to how the cows were used
Author-wise bibliometric analysis based on entropy.
No full textAuthor-wise bibliometric analysis based on entropy.</p
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